The present invention relates generally to the field of fuel cell fabrication and more particularly to fuel cell stack sintering and conditioning methods and apparatus.
Solid oxide fuel cells (“SOFC's”) are solid-state devices which use an oxygen ion conducting ceramic electrolyte to produce electrical current by transferring oxygen ions from an oxidizing gas stream at the cathode of the fuel cell to a reducing gas stream at the anode of the fuel cell. The oxidizing flow is typically air, while the fuel flow may be a hydrocarbon fuel, such as methane, natural gas, pentane, ethanol, or methanol. The fuel cell, operating at a typical temperature between 750° C. and 950° C., enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where the ion combines with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and/or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ion are routed back to the cathode side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit.
Fuel cell stacks are frequently built from a multiplicity of cells in the form of planar elements, tubes, or other geometries. Fuel cell stacks, particularly those with planar geometry, often use seals between electrolyte and interconnect surfaces to contain fuel and air at various locations within the stack. The stacks are often internally manifolded for fuel and/or air flow, and the ceramic electrolyte material may include internal openings or holes to accommodate fluid flow within the stack.
Fuel cell stack fabrication may require sintering of the fuel cells and separate conditioning of the fuel cells. During fuel cell stack sintering and conditioning the appropriate environment should be provided in the anode and cathode of the fuel cells. Previously, blowers and mass flow controllers have regulated the process gas flows from external gas supplies to the cell anode and cathode electrodes and the process gases have been exhausted after they pass through the fuel cell stack. Gas pre-heaters and heating coils have been used to regulate the heat input into the anode and cathode process gases. During the conditioning of fuel cell stacks, hydrogen from on-site generation or from off-site delivery is used to characterize the fuel cell stacks.
The fabrication of fuel cell stacks may involve a large number of complex process steps. Each process step needs to be carefully designed and process interactions considered in order to create a functioning product with high yield. In the fabrication process the fuel cell stacks and components may go through several thermal processes. The thermal gradients in those components can cause failures such as cracks or warping resulting in bad fuel cell stacks. Previous process controls have been implemented at the end user level. If a bad fuel cell stack was received by the end user, then the fabrication process was changed. If a good fuel cell stack was received by the end user, then the fabrication process continued unchanged. The previous process controls were slow to respond to problems, expensive, and did not increase fuel cell stack quality.